A medical image picked up by an X-ray CT apparatus does not have any unique CT value irrespective of the same type of a biomedical tissue due to influence of photon noise, system noise, a beam hardening effect or the like, and thus the dispersion of the CT value takes on a normal distribution having a representative value such as an average CT value or the like at the center thereof.
The X-ray absorption factor of each biomedical tissue when an examinee is irradiated with X-ray is dependent on the energy intensity of the irradiated X-ray. Accordingly, even when different types of biomedical tissues have X-ray absorption factors having the same level under irradiation of X-ray having some energy intensity, they have different X-ray absorption factors under irradiation of X-ray having another energy intensity.
There is a method of irradiating X-ray having different energy intensities by changing an image pickup tube voltage to pick up an image in order to discriminate the type of the biomedical tissue. For example, even in a case where different types of biomedical tissues have the same level of CT values on a reconstructed image when imaging is executed by setting the tube voltage to 120 kV, the contrast difference therebetween in the reconstructed image is increased when imaging is executed with the tube voltage of 80 kV or 140 kV, and thus the types of individual biomedical tissues can be discriminated.
According to a multi-energy type X-ray CT apparatus (MECT: Multi-Energy Computed Tomography), while one or more X-ray sources and one or more X-ray detectors disposed so as to confront the X-ray source(s) are rotated, an examinee is irradiated with X-ray from the X-ray source(s) to image the examinee disposed between the X-ray source and the X-ray detector. The multi-energy type X-ray CT apparatus is a medical imaging apparatus for detecting examinee-transmitted X-ray having two or more different energy intensities by the X-ray detector(s) to obtain transmitted X-ray information and reconstructed image information.
An apparatus for measuring bone density uses a DEXA (Dual Energy X-ray Absorptionmergy) method for irradiating X-rays having two different energy intensities and taking the difference therebetween to obtain the bone density. Furthermore, an X-ray CT apparatus uses a method of discriminating bone and a contrast blood vessel, contrast blood vessel and calcification in the blood vessel or the like by using examinee-transmitted X-ray information of two or more different energy intensities which are picked up by MECT (for example, see “Patent Document 1”, “Patent Document 2”).
Furthermore, there is known a method (for example, see “Non-patent Document 1”) for creating a map having CT values under high energy at the X-axis and CT values under low energy at the Y-axis on the basis of image information obtained by multi-energy imaging, and discriminating the type of a biomedical tissue by referring the map concerned. For example, when bone and contrast agent are discriminated from each other, a distribution area of bone and a distribution area of contrast agent on a map are sectioned from each other by a straight line. CT values of reconstructed image information obtained by multi-energy imaging are projected onto the map concerned, and a biomedical tissue located in the bone area is identified as “bone” while a biomedical tissue located in the contrast agent area is identified as “contrast agent”. The identified biomedical tissues are individually colored every biomedical tissue type and displayed with being superimposed on an original image.
Furthermore, there is a method for allocating colors in accordance with the CT value ratio between image information based on high energy intensity imaging and image information based on low energy intensity imaging (CT value based on low energy intensity/CT value based on high energy intensity) and displaying the colored image information while the image information is superimposed on an original image (for example, see “Non-patent document 2”).